mycobacterium

• Mycobacterium tuberculosis causes tuberculosis, with 1.4 million deaths in 2011 and 1/3 of the world population infected
• Mycobacterium tuberculosis is the most important pathogen among the mycobacterium group
• True tubercle bacilli are characterized by acid-fastness using the Ziehl-Neelsen technique due to the integrity of the waxy envelope
• Mycobacterium ulcerans causes necrotizing skin and soft tissue infections with ulcers
• Acid-Fast Bacilli


  • Rod-shaped, aerobic bacteria
  • Non-sporing
  • Do not stain readily/resist decolorization by acid or alcohol
  • Require special staining procedures using phenol-based, aryl methane dyes
  • Retain dyes due to high content of mycolic acids in cell wall
• Mycobacterium tuberculosis causes lower respiratory tract infections and is associated with AIDS
• There are over 125 species of known acid-fast bacilli, with Mycobacterium tuberculosis being the most prominent causing tuberculosis
• Mycobacterium tuberculosis is developing resistance to available drugs
• Cell wall of Mycobacteria

  Contains peptidoglycolipids, mycolic acids, other fatty acids, and waxes responsible for various characteristics like slow growth, acid fastness, and resistance to detergents and antibiotics
• Mycobacterium leprae causes Leprosy (Hansen’s disease)
• Mycobacterium avium-intracellulare (MAC) and other non-tuberculous mycobacteria frequently infect patients with AIDS and other immunocompromised persons
• Mycobacterium tuberculosis morphology: Thin, straight rods, grows coccoid and filamentous forms on artificial media, characterized by acid-fastness
• 95% ethyl alcohol containing 3% HCl (acid alcohol) quickly decolorizes all bacteria except mycobacteria
• Semi-synthetic Agar Media

  • Middlebrook 7H10 & 7H11
  • Inspissated Egg Media Löwenstein-Jensen
  • Broth Media Middlebrook 7H9 & 7H12
• Doubling time of the tubercle bacilli is 18 hours
• Humans and guinea pigs are susceptible to M. tuberculosis infection
• Components of Inspissated Egg Media

  • Defined salts
  • Glycerol
  • Complex organic substances Fresh eggs / egg yolks Potato flour
• Mycobacteria are resistant to drying and survive for long periods in dried sputum
• CULTURE

  1. Should include nonselective and selective media for primary culture

  2. SELECTIVE MEDIA contain antibiotics to prevent overgrowth of contaminating bacteria and fungi
• Components of Semi-synthetic Agar Media

  • Defined salts
  • Vitamins
  • Cofactors
  • Oleic acid
  • Albumin
  • Catalase
  • Glycerol
• Components of Broth Media

  • Middlebrook 7H9 and 7H12
  • Mycobacteria grow in clumps or masses Hydrophobic character of the cell surface plays an important role
• Mycobacteria are more resistant to chemical agents than other bacteria
• Variations in Mycobacteria

  • Colony appearance
  • Pigmentation
  • Virulence
  • Optimal growth temperature
  • Other cellular or growth characteristics
• Some atypical mycobacteria produce human disease indistinguishable from tuberculosis
• Constituents of Tubercle Bacilli

  • Lipids: Mycolic acids, Waxes, Phosphatides
  • Proteins
  • Polysaccharides
• Variation in Mycobacteria


  • Colony appearance
  • Pigmentation
  • Virulence
  • Optimal growth temperature
  • Other cellular or growth characteristics
• Fowl and cattle


  Are resistant to M. tuberculosis infection
• Phosphatides cause caseous necrosis, important among mycobacteria
• Phosphatides


  Cause caseous necrosis
• Polysaccharides' role in disease pathogenesis is uncertain but may induce immediate hypersensitivity
• Mycobacterium inhaled in droplets reach the alveoli


  Injected virulent bacilli survive for months or years in the normal host
• Cord factor inhibits leukocyte migration, aids in causing chronic granulomas, and serves as an immunologic adjuvant
• Analysis of lipids

  Done by gas chromatography to aid in classification of the organism
• Tubercle bacilli can survive for long periods in dried sputum
• Humans and capybaras are susceptible to M. tuberculosis infection


  Humans and guinea pigs are susceptible to M. tuberculosis infection
• Proteins elicit the tuberculin reaction and antibody formation
• Mycolic acids

  Partly responsible for acid-fastness of the organism
• Cord factor


  Responsible for the rope-like arrangement of the organism and linked to its pathogenicity
• Some mycobacteria cause surface lesions while others cause opportunistic infections
• M. tuberculosis and M. bovis


  Are equally pathogenic for humans